At the end of a GC that treated strings as a weak root for traversal, the string table is scanned for unreferenced entries, which are then deleted.
This can be time-consuming if the string table is large; this RFE is about improving the performance.
One way to do this is to parallelize the effort; JDK-8004888 implemented parallel traversal by claiming hash table buckets on a per-thread basis. A similar method could be applied in this case, and provided for all uses in a parallel collector (at least CMS, the parallel gcs, G1 full gc and during the G1 remark pause).